Spread the sugar evenly over the bottom of a medium sauté pan or wide medium saucepan, then set the pan over medium. As the sugar begins to melt at the edges, use a silicone spatula to push the liquefied sugar toward the center. Continue to cook, stirring gently, until all the sugar is melted, caramelizes to a deep amber color and begins to smoke, 3 to 5 minutes. Remove the pan from the heat, quickly add the butter and stir until melted. Gradually whisk in the cream and continue to whisk to dissolve any hardened bits of caramel.

Once the cream mixture is smooth, add the chocolate and stir gently until melted and smooth. Transfer the mixture to a large bowl and cool to room temperature, stirring occasionally. Whisk in the egg yolks.

In another large bowl, whip the egg whites until they hold stiff peaks. Using a silicone spatula, fold about ⅓ of the whites into the chocolate mixture, sprinkling in the fleur de sel. Fold in the remaining whites just until no streaks of white remain.

Divide the mousse into serving glasses or bowls, or transfer it to a large serving bowl. Cover and refrigerate for at least 8 hours or up to 24 hours.